1. Field of the Invention
The present invention relates to an ion beam irradiation device, which irradiates an ion beam onto an alignment layer for alignment of liquid crystal molecules during fabrication of a liquid crystal display, and a method of operating the same.
2. Description of the Related Art
In general, cathode ray tubes (CRT) have been widely used as display devices for displaying image information on screens. However, CRTs are problematic as they have large volumes and are heavy compared with their display areas.
Today, display devices are being used in desktop computers, notebook computers, wireless terminals such as cellular telephones and PDAs, automotive instrument boards, and electronic display boards. With the development of information communication technology, with the increasing ability to transmit high-capacity image information, the importance of a display device capable of processing such high-capacity image information is also increasing.
It is desirable for next generation display devices to be slim and lightweight, have a high brightness, a large screen size, low power consumption and low price. Liquid crystal displays (LCD) are accordingly gaining popularity as a flat panel displays capable of meeting the above requirements.
The LCD exhibits superior resolution compared with other flat panel displays and has rapid response speed. The response speed is comparable to that of the CRT.
The LCD uses the optical anisotropy and polarization of liquid crystals. Liquid crystal molecules with a thin and long structure have directionality and polarization. Hence, by applying an electric and magnetic field to the liquid crystal molecules, it is possible to control the alignment direction of the liquid crystal molecules.
To this end, by arbitrarily controlling the alignment direction of the liquid crystal molecules, the alignment of the liquid crystal molecules is varied and polarized light is modulated by the optical anisotropy of the liquid crystal, thereby displaying image information.
FIG. 1 is a plane view illustrating a pixel structure of a related art LCD.
Referring to FIG. 1, the LCD 100 includes a lower substrate 101, an upper substrate 102, and a liquid crystal layer interposed between the lower substrate 101 and the upper substrate 102.
The lower substrate 101 includes a first transparent substrate 111, a gate electrode 121 formed on the first transparent substrate 111, and a gate insulating layer 130 formed of silicon nitride (SiNx) or silicon oxide (SiOx) on the gate electrode 121 and the first transparent substrate 111.
An amorphous silicon active layer 141 is formed on the gate insulating layer 130, and an impurity-doped ohmic contact layer 151, 152 is formed on the active layer 141.
Source and drain electrodes 161 and 162 are formed on the ohmic contact layer 151, 152. The source and drain electrodes 161 and 162 form a thin film transistor together with the gate electrode 121.
A passivation layer 170 is formed on a resultant structure including the source and drain electrodes 161 and 162. The passivation layer 170 is formed of silicon nitride (SiNx), silicon oxide (SiOx) or organic insulator. A contact hole for electrical connection of the drain electrode 162 is formed in the passivation layer 170. A transparent conductive pixel electrode 181 is formed on pixel region of the passivation layer 170 and is electrically connected to the drain electrode 162 through the contact hole 171.
A first alignment layer 191 is formed on the pixel electrode 181. The first alignment layer 191 is formed of polyimide and is processed such that a surface thereof has a predetermined direction.
The gate electrode 121 is connected to a gate line, and the source electrode is connected to a data line. The gate line and the data line cross each other, thereby defining a pixel region.
The upper substrate 102 is disposed above the lower substrate 101. The upper and lower substrates are spaced apart by a predetermined interval.
A black matrix (BM) layer 120 is formed on the second transparent substrate 110. The BM layer 120 is formed at a portion corresponding to the thin film transistor formed on the first substrate 111 to prevent light from being transmitted in a region other than the pixel region.
A color filter layer 131 having red (R), green (G) and blue (B) color filters is formed beneath the black matrix layer 120. The red (R), green (G) and blue (B) color filters are sequentially repeated, and one color corresponds to a unit pixel region.
A transparent conductive common electrode 140 is formed beneath the color filter layer 131. A second alignment layer 150 is formed beneath the common electrode 140. The second alignment layer 150 is formed of polyimide and is processed such that a surface thereof has a predetermined direction.
The liquid crystal layer 190 is interposed between the first alignment layer 191 and the second alignment layer 150. The initial alignment state of liquid crystal molecules of the liquid crystal layer 190 is determined by the alignment direction of the first and second alignment layers 191 and 150.
Hereinafter, a process for forming the first and second alignment layers 191 and 150, which determine the initial alignment direction of the liquid crystal molecules of the liquid crystal layer, will be described in more detail.
First, the alignment layers are formed by coating a thin polymer film and alignment-processing the coated polymer film. A polyimide-based organic material is generally used as the alignment layer, and a rubbing method is used for aligning the alignment layer.
In the rubbing method, the alignment layer is formed by coating a polyimide-based organic film on a substrate, eliminating a solvent at a temperature of 60–80° C., curing the coated polyimide-based organic film at a temperature of 80–200° C., and rubbing the cured polyimide-based organic film using a rubbing cloth of velvet in a constant direction.
The rubbing method is easy and provides a stable alignment. Accordingly the rubbing method is amenable for use in mass production.
However, the rubbing method is problematic for a number of reasons; since the rubbing method is performed by directly contacting the alignment layer with the rubbing cloth, cell contamination due to particles being transferred from the rubbing cloth as well as fracture of TFT due to static electricity may occur. The rubbing method also requires an additional cleaning after the rubbing. Further, when the rubbing method is used to produce large screens (over about 27 inches), the alignment uniformity is severely degraded, thereby reducing the production yield of the LCDs.
To improve the drawbacks of the rubbing process, non-rubbing techniques that do not use such a mechanical rubbing have been proposed.
There are various non-rubbing methods, for example, a method using Langmuir-Blodgett (LB) film, an optical alignment method using UV irradiation, a method using a micro-groove formed by a photolithography process, a method using ion beam irradiation and the like.
The method using the ion beam has advantages in that it can solve the drawbacks of the mechanical rubbing method, can use old materials for the alignment layer, and can be applied to a large sized screen.
FIG. 2 is a schematic view of a related art ion beam irradiation device used for forming an alignment layer.
The related art ion beam irradiation device 260 is divided into three functional regions. Injection gas is ionized into ions and electrons form plasma in the first region 203. The ions are converted into a beam and are accelerated in the second region 206. The third region 211, or the ion beam irradiation region, ranges from a discharge point of the accelerated ion beam 210 to a substrate 220.
In the first region 203, the injected gas is ionized into ions. The ionized ions are then extracted, accelerated and irradiated onto the substrate 220. The ion beam irradiation device 260 is designed to irradiate the ion beam 210 onto the substrate 220 fixed to a holder 221 of a vacuum chamber 240.
The ion beam irradiation device 260 has an ion beam source 200 including a cathode 201, an anode 202, an ion beam extracting medium 204, and an ion beam accelerating medium 205. Also, the ion beam irradiation device 260 is further provided with the vacuum chamber allowing the ion beam generated by the ion beam source 200 to be moved in a straight line to the substrate 220 and irradiated onto the substrate 220. The ion beam irradiation device 260 is further provided with the holder 221 for fixing the substrate 220 such that the substrate maintains a predetermined angle within the vacuum chamber 240.
Although not shown in the drawings, the ion beam irradiation device 260 may be further provided, between the ion beam source 200 and the substrate 220, with a shutter so as to control when the ion beam 210 arrives on the substrate 220.
The ion beam source 200 generates ions and the ion beams 210. In other words, the injected gas is ionized by a voltage difference between the cathode 201 and the anode 202, thereby generating plasma including ions and electrons. The generated ions pass through orifices of the ion beam extracting medium 204 by an extracting electrode and are extracted as the ion beam.
The ion beam 210 extracted from the discharged plasma is accelerated by interaction of an electric field applied to the ion beam accelerating medium 205 and is then irradiated onto the substrate 220 at a predetermined angle.
The substrate 220 is disposed inclined with respect to the ion beam 210, thereby setting a desired pretilt angle on the alignment layer coated on the substrate 220.
Thus, the ion beam 210 generated by the ion beam source 200 is extracted in a normal direction of the ion beam source 200 and is then irradiated onto the substrate inclined at the predetermined angle θ1. The pretilt angle is determined by an irradiation angle θ2 of the ion beam 210. As shown, the irradiation angle θ2 is equal to the inclined angle θ1.
The irradiation angle θ2 represents an angle between the irradiation direction of the ion beam 210 and the normal direction of the substrate 220. The relation between the irradiation angle θ2 of the ion beam and the pretilt angle is shown in FIG. 3.
Referring to FIG. 3, the pretilt angle exhibits different characteristics depending on the irradiation angle of the ion beam. When the irradiation angle is between 40 degrees and 60 degrees, a maximum pretilt angle is obtained. In a range other than the above range, the pretilt angle is below 5 degrees.
Accordingly, to obtain a uniform desired pretilt angle in LCDs, the ion beam must irradiate the entire surface of the alignment layer of the substrate at the same energy and at a proper irradiation angle.
However, the related art ion beam irradiation device is not amenable to mass production, at least because the pretilt angle has to be reset each time the irradiated substrate is removed and a new substrate added. This increases the time needed to form the pretilt angle, resulting in a low throughput.